Apple released OS X 10.8 on Wednesday 25 July after development of one year .available in the Mac app store. if already have Snow Leopard or Lion , you can upgrade at  price of $19.99.

whats new : 

• Great iCloud integration, Messages, and social sharing,  brand new Notification Center
• New Messages app, which replaces iChat® and brings iMessage to the Mac
• Social sharing  easy to share links, photos, videos and other files quickly without having to switch to another app, and you just need to sign in once to use third-party services like Facebook, Twitter, Flickr and Vimeo .
• Game Center, which brings the popular social gaming network from iOS to the Mac
• Dictation, which allows you to dictate text anywhere you can type
• see all 200 new changes >>

you can upgrade from here : http://itunes.apple.com/us/app/os-x-mountain-lion/id537386512?ls=1&mt;=12
enjoy mountain lion